Transistor circuit providing vibrato effects for electrically operated musical instruments



Sept. 19, 1967 H. B. PADGETT 3,342,924

TRANSISTOR CIRCUIT PROVIDING VIBRATO EFFECTS FOR ELECTRICALLY OPERATED MUSICAL INSTRUMENTS Filed Jan. 15, 1964 w- 3 IN VENTOR jyezu yfl Pa e ATTORNEYS United States Patent TRANSISTQR CIRCUIT PROVIDING VIBRATO EF- FECTS FOR ELECTRICALLY OPERATED MUSI- CAL INSTRUMENTS Henry B. Padgett, Albuquerque, N. Mex., assignor to Byron Guse, trustee Filed Jan. 15, 1964, Ser. No. 337,883 Claims. (Cl. 84-125) ABSTRACT OF THE DISCLOSURE A vibrato system for insertion between a musical instrument and an amplifier, comprising an oscillator including two PNP type transistors having their collectors connected to the opposite ends of the primary winding of an audio type transistor transformer, a base biasing resistor connected between the base electrodes of the transistors and a center tap terminal of the transformer primary, a variable resistor connected between the center tap terminal and a reference voltage source, a capacitor connected across the secondary winding of the transformer to form a tank circuit therewith for establishing subaudio frequency oscillations, and an output circuit for the oscillator having a transistor serving as a variable impedance, the oscillation signals produced by the oscillator being applied through a limiting resistor to the base electrode of the transistor in the output circuit.

The present invention relates generally to transistor oscillators and frequency modulating systems, and more particularly to transistor oscillators and modulating systems employed for generating tremolo or vibrato signals for electrical musical instruments. Still more particularly, the present invention relates to a modulating device which may be inserted between an electronic amplifier and a musical instrument, or incorporated in an amplifier unit for a musical instrument for the production of a vibrato or tremolo effect.

It has been found by experience to be desirable when playing certain musical instruments such as an electrically amplified guitar, to modulate the resulting sounds with a vibrato or tremolo effect. This has been accomplished by electronic oscillators using various modulating techniques to create this effect. Previous tremolo vibrato units are generally bulky in size if not incorporated within a larger main amplifier and are not portable in the sense of small size, light weight, convenience of the operating controls or freedom from external power supply cords.

These facts impose limitations upon a musician using this type of equipment by restricting or limiting his position, as for example, while performing on stage, due to the necessity of remaining in the vicinity of such equipment in order to operate the controls, or require extra steps to be taken to operate this equipment, thus restricting his performance and ofttime's timing.

In addition, many amplifiers for musical instruments are currently in use which do not have a vibrato circuit.

Much of this equipment would be come obsolete due to the desirable vibrato circuit being omitted, thus depriving the owner of such an amplifier the opportunity to realize his initial investment or fully use the present equipment effectively.

. lightweight, fully portable unit having an extremely small Patented Sept. 19, 1967 HCC current drain during operation. The operating controls are within hand reach at all times since the preferred embodiment of this invention is attached directly to the electrical musical instrument with no intervening cords or separate parts, within a housing.

Other objects of my invention will become apparent from the following description, it being understood that the above general statements of the objects of my invention are intended to describe and not limit it in any way.

Referring to the drawing, the figure is a schematic view of tremolo means incorporated into an audio-amplifier according to the. present invention.

Referring more particularly to the accompanying drawing, reference numeral 1 denotes a PNP type transistor having an emitter electrode 2, a base electrode 3, and a collector electrode 4. Numeral 5 denotes a PNP type transistor having an emitter electrode 6, a base electrode 7 and a collector electrode 8. Both transistors 1 and 5 have their respective base electrodes connected to a source of negative direct current voltage 38, through base bias resistors 10 and 11, and the speed control, variable resistor 9. Transformer 12 is an audio type transistor transformer having a center tapped primary winding 13, and a secondary winding 14. The turns ratio, primary to secondary is 1:1. This transformer is preferably of the type having two separate windings on a single iron core, mutually coupled, with two end terminals on the secondary winding and two end terminals and a center tap terminal on the primary winding. Collectors 4 and 8 are connected to the end terminals of primary 13. Base electrodes 3 and 7 are connected to the respective end terminals of secondary 14, making this winding common to both bases. Capacitor 15 is connected in parallel with secondary Winding 14, establishing a parallel resonant circuit. The collector and base voltages are supplied by battery 38 through line 41, through variable resistor 9, from this point through bias resistors 10 and 11 to base electrodes 3 and 7 and from the center tap of the transformer primary Winding 13, to the collectors 4 and 8. This combination of parts constitutes the oscillator portion of FIG- URE 1.

Further referring to the figure and more particularly to the undescribed portion of this schematic view, resistor 16 connects base 7 of transistor 5, and base elec trode 19, of transistor 17, together to provide a path for oscillations from the oscillator described above, to the base of transistor 17 and performs the function of providing a minimum load for the oscillator as well as lim- .iting the amplitude of these oscillations applied to the base of transistor 17. This resistor may be replaced by a variable resistor if control over the percentage of modulation is desired. Transistor 17 is a PNP type, but may also be an NPN type, having an emitter electrode 18, a base electrode 19, and a collector 20. Emitter 18 terminates at a common ground point 39, and collector 29 connects to a signal carrying line 4-0. Signal carrying line 40 connects by means to the IN position of switch 21, identified as terminal 23. Transistor 17, having no direct current biasing or operating potential, responds only to the alternating current oscillations applied to base electrode 19. The combination of these parts constitutes the means of modulation of the signals appearing on line 40.

Capacitor 30 is an alternating current coupling and direct current blocking capacitor which serves to isolate transistor 17, collector electrode 20, from direct current voltages and serves to couple the signal output of transistor preamplifier 24 to signal line 40. Transistor 24 is a PNP type having an emitter electrode 25, a base electrode 26, and a collector electrode 27. Emitter 25 connects to ground 39. Bias potentials are established by collector resistor 29, one end being connected to power supply line 41, and by base bias resistor 28 which connects from base 26 to collector 27. This combination constitutes the preamplifier portion of the schematic view, with the input signals applied to the base electrode and the amplified signals removed at the collector. The operation is conventional for this type of circuit.

The input circuit consists of plug 34, resistor 32 and capacitor 31. The input signal arriving at plug 34 passes through resistor 32 which is a limiting and tone compensating resistor, serving also to isolate the rectifier action of transistor 24 from line 33, passes through capacitor 31, a direct current blocking capacitor to base 26 where these signals are amplified.

The output circuit consists of output jack 42, signal carrying lines 40 and 33, and IN/OUT switch 21 having terminals 22 and 23. The signals appearing on lines 40 and 33 are selected by switch 21, either in an unmodulated or modulated form, and appear at the output jack 42, from whence the signal can be removed for further amplification by a power amplifier.

The power supply providing a source of reference potential for the preamplifier and oscillator transistors consists of a battery 38 and a power switch 35. The common terminal of switch 35 is connected to a common chassis ground 39 and has an OFF position 36 and an ON position 37. Position 37 is a terminal to which the positive terminal of the battery is connected. This switch energizes the circuit by applying a potential between a common ground 39, and power supply line 41 which also is common to the amplifier and oscillator circuit.

The combination of all the above-described parts is assembled as acommon, integral unit.

Referring now to the operation of the oscillator, base bias resistors 10 and 11 establish a forward bias condition for transistors 1 and 5 and differ in value from each other, for example, 220K ohms and 22K ohms. Different values for base bias voltages are thus established, insuring that conduction occurs to a greater degree in one transistor at all times, further insuring a positive start to the oscillator and permitting the use of transistors having identical or widely different characteristics. A common tank circuit between base electrodes 3 and 7 of transistors 1 and 5, consisting of secondary winding 14 and capacitor 15, accelerates the switch-off interval of each transistor by applying an opposite polarity pulse to the base electrodes alternately to each transistor, this pulse, caused by capacitor 15 discharging each cycle, and the amplitude of this pulse tending to overcome the fixed bias established by resistors 10 and 11, thus driving each base rapidly into the cutoff portion of their characteristic curves.

Transistors 1 and 5 combining transformer 12, comprise a regenerative feed-back oscillator with complementary actions. Saturation occurs alternately in each transistor of the oscillator at a rate determined by capacitor and transformer 12 inductance. Base bias resistors 10 and 11 are connected to the center tap of transformer primary 13 and to one end of the variable potentiometer 9. The center arm terminal of resistor 9 connects to a source of potential appearing on line 41. Varying the value of resistor 9 changes the bias voltages applied to base electrodes 3 and 7, and collector electrodes 4 and 8, resulting in current changes at the collectors, causing in turn the frequency of the oscillator to increase with an increase of these currents, thus acting to control the speed of the tremolo effect. The oscillator delivers high purity sine waves of great stability to temperature and transistor parameters.

The output of the described oscillator is removed from base electrode 7 and applied through resistor 16, which may be variable in value, to the base 19 of transistor 17. Transistor 17 has no applied direct current potentials and responds only to the oscillator sine wave amplitude, polarity and current changes. A PNP type transistor is indicated on the schematic view for transistor 17, but may be an NPN type. With transistor 17 having applied to base electrode 19 the negative half cycle of the sine wave from the oscillator, transistor 17 views this negative half cycle as an instantaneous, short duration, reverse bias at its base. The amplitude of the applied oscillator sine wave drives transistor 17 into the saturation region of its characteristic curve. Under this condition transistor 17 has a low internal impedance appearing between collector and emitter. Connecting collector electrode 20 to signal carrying line 40 will result in the amplitude of the signals appearing on this line 40 being reduced due by the low impedance appearing across the collector and emitter junction of transistor 17. During the next half cycle of the oscillator sine wave, the applied instantaneous polarity is reversed, restoring the impedance of transistor 17 to a high value, which in turn allows the signals on line 40 to be restored to normal amplitude. These amplitude variations caused by transistor 17 varying in impedance, occur at the oscillator frequency and appear on signal carrying line 40 as modulation, which when amplified and projected by a speaker appear as a tremolo effect. The speed of this tremolo is varied by resistor 9 and the depth or percentage of modulation is controlled by the value of resistor 16.

During the act of modulating the signals on line 40, part of these signals are lost across the varying impedance of transistor 17, resulting in the average signal level which appears at output jack 42 being reduced if such modulation is applied directly to a signal source having no previous amplification. Therefore, incorporated in this device is a stage of preamplification designed to restore the average signal loss due to modulation. Transistor 24 operates as a class A amplifier, biased by base resistor 28 and collector resistor 29. Transistor 24 is a PNP type having an emitter electrode 25, a base electrode 26 and a collector electrode 27. Capacitor 30 is a blocking capacitor for transistor collector 17 and couples the output of transistor 24 to signal carrying line 40. Battery 38 supplies a negative voltage to power line 41. In series with the signal input plug 34 is an amplitude limiting resistor 32 and a blocking capacitor 31. Line 33 connects at the junction of plug 34 and resistor 32 and terminates at terminal 22 of the tremolo IN/OUT switch 21, which is a single pole double throw switch. Switch 21 connects from its common terminal to the output jack 42 where the input signal from plug 34 is made available for further use.

The operation of transistor preamplifier 24 and switch 21 is as follows. The relatively high frequency signals applied to plug 34 pass through resistor 32, blocking capacitor 31 to the base electrode 26 of transistor preamplifier 24. Transistor 24 amplifies these signals which pass through capacitor 30, line 40, and appear at the IN position 23 of switch 21. With switch 21 in the IN position, the signals appear at the output jack 42 containing modulation as acted upon by transistor 17. With switch 21 in the OUT position 22, the signals from plug 34 pass directly to output jack 42, presenting signals at the jack 42 having no modulation. The above combination of parts constitutes the input circuit, the output circuit and the preamplifier circuit.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the general arrangement and of the details of construction which are specifically illustrated and described, may be resorted to without departing from the true spirit and scope of the invention. as defined in the appended claims.

What is claimed is.

1. A vibrato system for insertion between a musical instrument and an amplifier comprising an oscillator including two transistors, each having a base electrode, an

. emitter electrode and a collector electrode, a transformer including a primary winding having two end terminals and a center top terminal and a secondary winding having two end terminals, a capacitor connected in parallel with said secondary winding forming a tank circuit therewith for establishing sub-audio frequency oscillations, means connecting the end terminals of said secondary winding to said base electrodes, means connecting the end terminals of said primary winding to said collector electrodes, a base biasing resistor connected between each of said base electrodes and said center top terminal, a variable resistor connected between said center top terminal and a source of reference potential for controlling the frequency of oscillations by varying the voltage applied through said variable resistor to said base and collector electrodes, a signal carrying line for receiving electrical signals derived from the musical instrument and applying them to an output terminal, an output circuit for said oscillator including a transistor serving as a variable impedance and having a base electrode, an emitter electrode and a collector electrode, and means for applying oscillation signals produced by said oscillator through a limiting resistor to the base electrode of said last-mentioned transistor.

2. A vibrato system for insertion between a musical instrument and an amplifier comprising an oscillator including two transistors, each having a base electrode, an emitter electrode and a collector electrode, a transformer including a primary winding having two end terminals and a center top terminal and a secondary winding having two end terminals, a capacitor connected in parallel with said secondary winding forming a tank circuit therewith for establishing sub-audio frequency oscillations,

means connecting the end terminals of said secondary winding to said base electrodes, means connecting the end terminals of said primary winding to said collector electrodes, a base biasing resistor connected between each of said base electrodes and said center top terminal, a variable resistor connected between said center top terminal and a source of reference potential for controlling the frequency of oscillations by varying the voltage applied through said variable resistor to said base and collector electrodes, a signal carrying line for receiving electrical signals derived from the musical instrument and applying them to an output terminal, a modulating transistor serving as a variable impedance for modulating said electrical signals including a base electrode, an emitter electrode and a collector electrode, means for coupling oscillation signals produced by said oscillator through a limiting resistor to said base electrode of said modulating transistor, and said means for connecting the collector electrode of said modulating transistor to said signal carrying line for modulating the signals received thereby as a function of said oscillation signals.

3. In a vibrato system, the combination defined in claim 2, wherein said electrodes of said modulating transistor have no source of direct current potential, means connecting the emitter electrode of said modulating transistor to a common ground, said oscillator providing said modulating transistor with variable biasing currents and 6 voltages and said collector connection to said signal carrying line providing a shunt path to said common ground through the variable internal impedance of said modulating transistor.

4. In a vibrato system, the combination defined in claim 2, including an input circuit adapted to be coupled to a musical instrument to receive audio-frequency electrical input signals, a transistor pre-amplifier having a base electrode, an emitter electrode and a collector electrode, means connecting the base electrode of said pro-amplifier to said input signals, and means coupling the collector electrode of said pre-amplifier through a blocking capacitor to said signal carrying line connected to the collector electrode of said modulating transistor, said pre-amplifier having sufficient gain to compensate for the average signal amplitude loss caused by modulation of the electrical signals on said signal carrying line by said modulating transistor.

5. A vibrato system for insertion between a musical instrument and an output adapted to be coupled to an amplifier comprising an oscillator for generating subaudio frequency oscillation signals for modulating electrical signals derived from the musical instrument, means for varying the frequency of oscillation of said oscillator, an input circuit to receive signals directly from the musical instrument, a transistor pre-amplifier having a base electrode connected to said input circuit, an emitter electrode, and a collector electrode, a signal carrying line connected to an output terminal and through a blocking capacitor to the collector electrode of said transistor preamplifier to receive the electrical signal from the preamplifier, a modulating transistor serving as a variable impedance for modulating said electrical signals including a base electrode, an emitter electrode and a collector electrode, means for coupling said oscillation signals from said oscillator through a limiting resistor to said base electrode of said modulating transistor to vary the internal impedance thereof, and means for connecting the collector electrode of said modulating transistor directly to said signal carrying line for modulating the signals thereon as a function of said oscillation signals, said modulating transistor electrodes being free of connection with any source of direct current potential, said preamplifier having sufficient gain to compensate for the average signal amplitude loss caused by modulation of the electrical signals on said signal carrying line by said modulating transistor.

References Cited UNITED STATES PATENTS 3,076,370 2/1963 Williams 841.25

ARTHUR GAUSS, Primary Examiner.

J. BUSCH, Assistant Examiner. 

1. A VIBRATO SYSTEM FOR INSERTION BETWEEN A MUSICAL INSTRUMENT AND AN AMPLIFIER COMPRISING AN OSCILLATOR INCLUDING TWO TRANSISTORS, EACH HAVING A BASE ELECTRODE, AN EMITTER ELECTRODE AND A COLLECTOR ELECTRODE, A TRANSFORMER INCLUDING A PRIMARY WINDING HAVING TWO END TERMINALS AND A CENTER TOP TERMINAL AND A SECONDARY WINDING HAVING TWO END TERMINALS, A CAPACITOR CONNECTED IN PARALLEL WITH SAID SECONDARY WINDING FORMING A TANK CIRCUIT THEREWITH FOR ESTABLISHING SUB-AUDIO FREQUENCY OSCILLATIONS, MEANS CONNECTING THE END TERMINALS OF SAID SECONDARY WINDING TO SAID BASE ELECTRODES, MEANS CONNECTING THE END TERMINALS OF SAID PRIMARY WINDING TO SAID COLLECTOR ELECTRODES, A BASE BIASING RESISTOR CONNECTED BETWEEN EACH OF SAID BASE ELECTRODES AND SAID CENTER TOP TERMINAL, A VARIABLE RESISTOR CONNECTED BETWEEN SAID CENTER TOP TERMINAL AND A SOURCE OF REFERENCE POTENTIAL FOR CONTROLLING THE FREQUENCY OF OSCILLATIONS BY VARYING THE VOLTAGE APPLIED THROUGH SAID VARIABLE RESISTOR TO SAID BASE AND COLLECTOR ELECTRODES, A SIGNAL CARRYING LINE FOR RECEIVING ELECTRICAL SIGNALS DERIVED FROM THE MUSICAL INSTRUMENT, AND APPLYING THEM TO AN OUTPUT TERMINAL, AN OUTPUT CIRCUIT FOR SAID OSCILLATOR INCLUDING A TRANSISTOR SERVING AS A VARIABLE IMPEDANCE AND HAVING A BASE ELECTRODE, AN EMITTER ELECTRODE AND A COLLECTOR ELECTRODE, AND MEANS FOR APPLYING OSCILLATION SIGNALS PRODUCED BY SAID OSCILLATOR THROUGH A LIMITING RESISTOR TO THE BASE ELEECTRODE OF SAID LAST-MENTIONED TRANSISTOR. 